There is something called auto-commit, when you supply more than one query delimited by ; semicolon all-or-none is done if one fails. No need for BEGIN;COMMIT;ROLLBACK when doing one query. its logic to mee pg_affected_rows() returns affected rows and if you want to do 2 queries apart from each other.. do a BEGIN and then 1 and get pg_affected_rows() then do 2 and get pg_affected_rows() and then finally do COMMIT;

That's not quite true, I've been able to execute multiple queries in a single call just fine. In stead, it has to do with the fact this function returns the affected rows for the last executed query, not the last set of queries specified to a single call to pg_query.